As it is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized sheet packaging material.
A typical example of this type of packages is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated sheet packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material, and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As it is known, packages of this sort are produced on fully automatic packaging machines from a continuous web of packaging material, which may be cut to form blanks or longitudinally sealed to form a tube of packaging material.
In the latter case, which will be referred to hereinafter without loss of its general sense, the web of packaging material is loaded into the packaging machine in the form of a reel, from which it continuously unwinds, and is fed to an aseptic chamber for sterilization, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is later vaporized by heating, and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity.
The sterilized web is then folded into a cylinder and sealed longitudinally to form, in known manner, a continuous, vertical, longitudinally sealed tube, which forms an extension of the aseptic chamber; the tube of packaging material is filled continuously with sterilized or sterile-processed pourable food product and then fed to a form-and-seal unit for forming the individual packages.
The form-and-seal unit comprises pairs of jaws that are cyclically brought into contact with the tube to grip and seal it at equally spaced cross sections and to form so-called “pillow packs” connected to the tube by transverse sealing strips.
The pillow packs are then separated from the tube by cutting the relative transverse sealing strips, and are conveyed to a folding station where they are folded mechanically to form respective finished, e.g. parallelepiped-shaped, packages.
To allow folding of the web packaging material both during forming and final folding, bend or fold lines, i.e. creased or weakened lines, defining a so-called “crease pattern” are embossed on the packaging material at the production line (creasing operation).
More specifically, the web packaging material is produced in converting plants where a paperboard, a prelaminated paperboard or the like, typically including a paper layer covered on one side with layers of heat-seal plastic material and barrier material, is subjected to a number of successive processing operations including the above-mentioned creasing operation.
Typical examples of successive processing operations performed on the web at the converting plant are:                printing a repeated design pattern, usually made in multiple successive print units, e.g. one unit for printing each colour;        embossing a repeated pattern of bend or fold lines (crease pattern);        perforating, scoring or cutting the web material through mechanical or laser devices; and        forming a further heat-seal plastic material layer on the printed side.        
In order to assure that all discrete operations both at the converting plant and in the packaging machine are in register with one another, i.e. made at correct relative positions on the web, register marks must be provided on the web.
According to a known process, this aim is achieved by printing a first and a second register mark on the web at the first print unit. The first register mark is used in the successive steps of the converting process to determine the actual position of the web in order to perform the successive operations, such as printing the remaining colours, creasing, and laser-processing, mechanical perforating, hole punching, etc., if any. The second register mark is used in the packaging machine in order to control the feed of the tube of packaging material and the forming operations thereon.
EP-B-0357841 discloses a method of providing score lines in a web packaging material through a laser device, which is triggered by the detection of a recurring printed register mark on said material.
Printed marks to detect the longitudinal position of the web material have been used for many years, are simple to produce and easy to read; in fact, the printed mark is simply a portion of the package design and thus does not bring about any additional cost. However, being a portion of the printed design means that the register mark itself is, for instance, not perfectly in register with the respective bend or fold lines, because printing and creasing steps are two different successive operations in web material production and, although relative shifts between the two operations are kept to a minimum, there are inherent tolerances in the production process (print-to-crease tolerances). This may cause problems in successive packaging operations, wherein the packaging material is positioned according to the printed marks in order to be folded at the bend or fold lines.
In general, the use of a printed register mark for successive operations implies that such operations are performed with a position error, depending on the process tolerances, which is referred to the printed mark. This means that any result of a successive operation (printed design, crease pattern, laser pattern, etc.) may have a positive or negative position error with respect to a theoretical position determined by the printed mark; the absolute value of the error is comprised within a maximum value depending on the process tolerances. In case two successive operations which have a functional impact when forming and filling packages, such as the crease pattern and the laser pattern, happen to be subjected to errors in opposite directions, the tolerance chain may produce a relative error between such operations which is up to the sum of the tolerance widths of each single operation.
To eliminate the above-mentioned relative error between the operations for making the crease pattern and the laser pattern, it has been proposed in U.S. Pat. No. 6,046,427 to detect the position of the bend or fold lines on the packaging material for triggering the laser device. This method allows to avoid generating the print-to-crease tolerances but introduces other problems.
In particular, if a bend or fold line is used as a “register mark”, the position of the web material in a given, e.g. longitudinal, direction can be detected provided that such bend or fold line is correctly identified with respect to the other bend or fold lines forming the crease pattern. This would require to use an additional reference code, e.g. a printed code, to trigger a “reading window”, or alternatively extremely complicated sensors.
Moreover, such sensors can be used to detect bend or fold lines on flat material in converting operations, but are not suitable for use in a tube-fed packaging machine in which there is:                the physical pulsating movement of the pourable food product within the tube of packaging material during filling and packaging operations due to internal pressure changes;        the vertical displacements in the tube-feed direction;        the horizontal displacements in the horizontal “tube twisting” direction; and        the arduous conditions of the working environment in which the sensors would have to operate.        
Therefore, it is necessary to provide additional printed marks adapted to be read more easily in the tube-fed packaging machine or to use different optically readable indicia, such as the position of a web edge, or prelaminated holes for opening devices, or the longitudinal seal of the tube; however, these indicia are difficult to detect for the reasons above-mentioned, and in any case require dedicated sensors.
To sum up, although effective to eliminate the “print-to-crease” tolerances, the proposal contained in U.S. Pat. No. 6,046,427 still requires two detection systems, one for printed or optically detectable indicia, the other for bend or fold lines.